In a conventional time division multiple access (TDMA) communication system, such as a communication satellite relay link, a number of transmitting stations share a common carrier in a time division multiplex mode. The information is transmitted to the satellite in reoccuring increments of time commonly referred to as frames. Each frame is divided into subframe intervals, one for each of the earth stations transmitting on that carrier (See FIG. 1). Each earth station in turn transmits a burst of information during its subframe interval.
The satellite receives the information transmitted on each carrier, converts it to a downlink frequency which is transmitted back to earth as an echo of the uplink signal. The earth stations then receive this signal and divide it into frames and subframes to extract the information transmitted by each of the earth stations.
The time at which each earth station transmits its information is very critical so that its transmission does not overlap or interfere with the signals from the earth stations transmitting during adjacent subframes. The synchronization of the earth station signals is complicated by the difference in transmission distances to and from the satellite, clock instabilities at each station, and the orbital drift of the satellite. In a conventional TDMA communication system each of the transmitting earth stations receives the downlink signal and from that signal uses the timing relationships between its signal and the adjacent ones to determine whether its transmission is properly timed with respect to the adjacent subframes.
An alternative communication system has been proposed in which a number of uplink carriers are transmitted in a conventional TDMA mode. The satellite then time division multiplexes these various uplink channels into a single downlink channel. For example, the uplink channel could have a 72 MHz bandwidth which would permit six frequency division multiplexed carriers each having a ten Megabit data rate. Each individual carrier is in turn shared by a number of transmitting earth stations in a time division multiple access mode. With TDMA, the actual transmission on each carrier consists of a burst of data at a ten megabit rate from one earth station followed by some guard time, then followed by a second burst from a second transmitting earth station and so on until all of the participating earth stations sharing the uplink carrier have bursted their information. This would complete one transmission frame and another frame would then begin repeating the cycle.
At the satellite each of the six carriers is received and demodulated to extract the transmitted information. The information from each of the six carriers is stored temporarily in separate buffers. Data from each of the six uplink carriers is then time division multiplexed onto a single carrier at a higher data rate (e.g. 60 Megabits). This higher rate is equal to the sum total of the uplink data rates from the six individual carriers. During the transmission of the downlink signal each buffer sequentially empties its contents into the downlink signal in a conventional time division multiplex (TDM) transmission scheme.
In this type of regenerative subtransponder satellite communications system, the individual transmitting earth stations no longer receive a signal from the satellite which can be used to synchronize and time its transmission. This regenerative system has converted each of the six individual uplink carriers into a single high speed downlink carrier in which the individual timing relationships for the various uplink signals have been lost. In this system the downlink is completely divorced from the uplink because the individual data streams transmitted to the satellite are demodulated on board the satellite and the data is packed into buffers. Hence, the normal TDMA synchronization cannot be used.